There have been many devices in use for driving loads such as LEDs, utilizing a dc-dc conversion type power supply circuit adapted to provide an output voltage different from an inputted power supply voltage. A typical load driving device has a power supply circuit that generates a predetermined output voltage and an output current for driving a load, as disclosed in Japanese Patent Application Laid Open No. 2001-313423. For this purpose, the level of the output voltage or the output current supplied to the load is measured to establish a detection voltage or detection current, which is fed back to a control circuit of the power supply circuit.
In such conventional load driving device, the detection voltage is obtained by dividing the output voltage in a voltage dividing circuit having a high resistance. The detection current is obtained by detecting the potential drop across a resistor (referred to as voltage detection resistor) connected in series with the load, whereby the load current flows through the resistor. The detection voltage (or detection current) is compared to a reference value so that the output voltage (current) outputted from the power supply circuit is controlled based on the comparison.
In a portable electronic device such as a cellular phone, the load current is sometimes increased or decreased within a permitted range in response to a request made during service. For example, when the load is a light emitting diode (LED), a request is made to regulate the luminance of the LED to an arbitrary level.
In such an example, the voltage detection resistor connected in series with the load will increase energy loss when the load current is increased. Therefore, the overall efficiency of the electronic device that includes a power supply circuit and a load disadvantageously drops when the load current becomes large (i.e., during a heavy duty).
In another example, a request is made to drive one load with a constant current and at the same time to drive another load with a voltage above a predetermined voltage. In such an example, conventionally it is necessary to provide a further appropriate power supply circuit to meet individual use conditions, which requires additional space and cost for the power supply circuit and load.
It could, therefore, be helpful to provide a load driving device having a dc-dc conversion type power supply circuit that generates an output voltage by converting a power supply voltage (an input voltage), the power supply circuit capable of adjusting the magnitude of the load current within a predetermined range while avoiding the energy loss caused by an increase in the load current, thereby enabling efficient driving of the load.
It could also be helpful to provide a portable apparatus equipped with such a load driving device.
It could further be helpful to provide a load driving device having a dc-dc conversion type power supply circuit that generates an output voltage by converting a power supply voltage, the power supply circuit capable of driving a multiplicity of loads having different use conditions, including at least one constant-current type load and another type of load, and capable of adjusting the magnitude of the load current supplied to the constant-current type load within a predetermined range while maintaining the output voltage to another type of load above a predetermined voltage.
It could still further be helpful to provide a portable electronic apparatus equipped with such a load driving device.